Regulatory mutants which affect attenuation of the threonine (thr) operon will be isolated and studied. Recently developed in vitro mutagenesis techniques will be employed to direct mutations into specific sites in the thr regulatory region. Mutants will be characterized both by DNA sequencing and by their regulatory effects in vivo. The positions of the mutations and their effects on regulation will supply information on the relationship between secondary RNA structure and transcription termination. Another aspect of the proposal is concerned with generating mutations in the recombination site (att site) used by bacteriophage Lambda during site-specific recombination. Mutations will be induced in vitro and characterized by their effects on recombination in vivo and by their interactions with Lambda integrase (int) protein in DNA protection and binding assays.